Hello Dario and friends! Here is a webpage I made explaining the Atari 2600! You can also download the Atari 2600 MFA which features that flickering limitation when there are more than 2 sprites on the same scanline! Have fun creating Atari 2600-like games!
Btw, Yes! I had also been using Ultimate full screen! it is a blessing! but as of the time of this posting, it seems that it is not needed anymore with Fusion 2.5+ as they have a proportional resize with black side-bars, which is exactly how retro games were displayed on a wider screen, by Mame (for example), so the pixels are not distorted! BUT for Atari 2600 games, the screen does need a "double-wide" stretch, because its pixels were "2x1" unless you stacked 2 pixels on top of each other! see my page for more on this!
So for an Atari 2600 mfa, you can go 2 ways:
1- have a 160x192 canvas which needs stretching (but your graphics will be original in size), or
2- have a 320x192 canvas (no stretching needed and you will need to draw sprites double-wide on your paint program for later export)